The present invention relates to a method for protecting a copper surface, particularly in the manufacturing of electronic circuit boards.
The use of tin base solder alloys is common in electronic applications, particularly in the manufacturing of printed circuit boards (PCB), for assembly of components onto the boards, providing mechanical and electrical connection. These tin solder alloys are useful in joining integrated circuit chips to chip carriers and substrates, joining chip carriers to substrates, and joining circuitization lands and pads in multilayer printed circuit boards.
In the manufacturing of a microelectronic package, it is common practice to attach a component onto a printed circuit board or the like, for example by surface mounting utilizing a solder connection. For this purpose, the board features a circuit trace including a pad that constitutes a first surface for the connection; similarly, the component includes a second surface, for example a contact.
The interconnection method comprises the steps of applying a solder alloy on the Cu substrate, typically onto the pad included in the printed circuit board.
The electronic components to be joined with the board are then brought into contact with the solder layer. The solder alloy is heated to cause the solder alloy to melt and reflow; heating may be by vapor phase reflow, infrared reflow, laser reflow, or the like. Upon cooling, the solder alloy resolidifies and bonds to the surfaces to complete the connection. The solder connection not only physically attaches the component to the board, but also electrically connects the trace on the board and the contact of the component to conduct electrical current to and from the component for processing.
The oxidation of a copper surface exposed to the air is a well known problem, not only in the manufacturing of electronic modules. The oxidation of copper pads of a PCB affects the solderability of the copper surface with the tin based alloy and this can cause problems in the assembly of the electronic module, particularly when the PCB is exposed to air for a prolonged period before being processed. For this reason the PCB is usually treated with chemical solutions which protect the copper from oxidation.
One of the known techniques used for protecting copper surfaces from corrosion, especially in the field of electronic circuit boards, is the Organic Solderability Preservative coating as described for example in xe2x80x9cCorrosion Protection of Copper Using Organic Solderability Preservativesxe2x80x9d by I. Artaki et al. Circuit World Vol., 19 No. 3, 1993, pages 40-45. These organic coatings are usually based on azole or its derivatives. Azoles react with metallic copper forming a film which helps to inhibit copper oxidation without compromising the solderability of the copper surface. U.S. Pat. No. 3,295,917 discloses inhibiting copper corrosion by coating with benzotriazole. U.S. Pat. No. 3,933,531 discloses a preservative treatment with 2-alkyl imidazoles. U.S. Pat. No. 4,373,656 discloses a method for preserving a copper surface by immersing in imidazole.
As mentioned above, it is known to bathe the PCB in a solution containing BenzoTriAzole (BTA). A commercially available BTA based product is, for example, Entek56 produced by Enthone-Omi Inc.
However the prior art treatments are not always satisfactory, especially during the soldering process when the PCB undergoes high temperature processing.
Therefore an improved treatment for the copper surface would be highly desirable.
It is an object of the present invention to provide a technique which alleviates the above drawbacks.
According to the present invention we provide a solution for preserving a copper surface of an electronic module, the solution containing at least one compound selected from the family of the azoles, characterized in that it further comprises a zinc salt.
Further, according to the present invention we provide a method for protecting a copper surface of an electronic module comprising the step of immersing the copper surface in a solution containing at least one compound selected from the family of the azoles, characterized in that it further comprises a zinc salt.
Also, according to the present invention, we provide a method for soldering a metallic component on a copper surface with a tin base alloy, the method comprising the step of pretreating the copper surface with a solution as described herein.
The addition of the zinc (Zn) salts in the pretreatment solution gives substantial improvements to the copper (Cu) surface characteristics. The Cu surface oxidation decreased, both during soldering and the exposure to atmosphere; the Cu surface wettability is substantially improved, even after long time at relatively high temperature; and the adhesion of the tin (Sn) solder alloy is increased.
The decrease of oxidation of the Cu surface with time and temperature could be interesting and appreciated in many other applications, in fields different from the electronics industry, e.g. the treatment of copper roofs and panels in the building industry for maintaining the original copper aspect; other possible applications cover sanitary or hydraulic uses for protecting copper pipes.
The present invention takes advantage of the fact that Zn forms a complex with BTA, in presence of Cu increasing the stability of layer adsorbed on the surface and giving to the interface the advantages described above.